It is known from our U.S. Pat. No. 4,122,672 to provide a deep buzz-saw tone noise liner directly upstream of a fan in an intake duct of a turbofan gas turbine engine to reduce low frequency buzz-saw noise levels and to provide a shallower acoustic liner upstream of the buzz-saw tone noise liner in the intake duct of the turbofan gas turbine engine.
Buzz-saw noise, also called multiple tone noise or combination tone noise, occurs when the tips of the fan blades of a fan of a turbofan gas turbine engine rotate with supersonic velocities. The spectrum of buzz-saw noise contains energy in a wide range of harmonics of the engine rotation frequency; as opposed to blade passing frequency (BPF) harmonics when the tips of the fan blades of a fan of a turbofan gas turbine engine rotate with subsonic velocities. The source of the noise is the rotor alone noise which is now cut-on and this propagates up the intake duct in the form of shock waves and expansion fans; an N wave pattern. Because of slight differences in fan blades, the shocks are different from fan blade to fan blade and this is why the spectrum contains harmonics of the engine rotation frequency. Acoustic levels are typically high, 180 dB near the fan.
However, the buzz-saw tone noise propagates non-linearly in an upstream direction from the fan within the intake duct and there may be a large exchange of energy between different tone frequencies. These tones are multiples of the engine order frequencies and the deep buzz-saw tone noise liner is designed to attenuate a certain range of low order engine order frequencies, which are particularly prone to transmit through the wall of an aircraft cabin. However, because the buzz-saw tone noise propagation is non-linear, noise may leak back into these sensitive frequencies after the noise has passed the deep buzz-saw tone noise liner so that the effectiveness of the buzz-saw tone noise liner at the sensitive frequencies is reduced. This is particularly true if the engine order tone level is initially low due to liner attenuation.